In order to make an air-fuel ratio of gas mixture supplied into an internal combustion engine equal to a target air-fuel ratio, it is required to accurately estimate quantity of air which is introduced into a cylinder of the internal combustion engine (hereinafter, referred to as “cylinder intake-air quantity”). In a steady operation state where an opening of a throttle valve remains substantially unchanged, the cylinder intake-air quantity is accurately estimated based on a value output from an air flow sensor disposed in an intake passage of the internal combustion engine. Meanwhile, in a transition-operation state where the opening of the throttle valve rapidly varies, it is difficult to accurately estimate the cylinder intake-air quantity based on the value output from the air flow sensor, because time response characteristic of the air flow sensor is not sufficient. Therefore, it is required to estimate the cylinder intake-air quantity with better response than a case using the air flow sensor.
Further, fuel is generally injected before an end timing of an intake stroke. Meanwhile, the cylinder intake-air quantity is fixed (or becomes definite) at the end timing of the intake stroke (at an intake valve-closing timing). Therefore, the cylinder intake-air quantity has not become definite at a fuel-injection timing. Accordingly, in order to determine a fuel injection amount which makes the air-fuel ratio of the gas mixture to be formed in the cylinder equal to the target air-fuel ratio, it is required to estimate “the cylinder intake-air quantity at the intake valve-closing timing” at a predetermined timing before the fuel injection (i.e., before the end timing of the intake stroke).
As one of cylinder intake-air quantity estimation apparatuses which can satisfy such demands described above, Japanese Patent Application Laid-Open (kokai) No. 2001-41095 discloses an apparatus for estimating cylinder intake-air quantity at a future point in time later than a present point in time by use of a physical model which is modeled on a behavior of air flowing through the intake passage of the internal combustion engine.
According to the physical model which the disclosed apparatus uses, the estimated cylinder intake-air quantity at the future point in time later than the present point in time is represented by equations using pressure of air at a position upstream of the throttle valve (throttle valve upstream air) at the future point in time and temperature of the throttle valve upstream air at the future point in time. Therefore, unless the pressure and the temperature of the throttle valve upstream air at the future point in time are accurately estimated, the cylinder intake-air quantity at the future point in time cannot be accurately estimated.
Incidentally, since the above-mentioned internal combustion engine, to which aforementioned conventional apparatus is applied, is a naturally-aspirated engine, the pressure and the temperature of the throttle valve upstream air are approximately equal to those of atmosphere (or air). Further, the pressure and the temperature of atmosphere remains substantially unchanged within a short period of time from the present point in time to the future point in time when the cylinder intake-air quantity is to be estimated. Accordingly, the above-mentioned conventional apparatus accurately estimates the cylinder intake-air quantity at the future point in time, by employing the pressure of atmosphere and an intake air temperature detected by a temperature sensor disposed in the intake passage as the pressure and the temperature of the throttle valve upstream air, respectively.
However, if an internal combustion engine has a supercharger and an intercooler in order to improve its maximum output etc., air in the intake passage is compressed by the supercharger and is cooled by the intercooler. Therefore, the pressure and the temperature of the throttle valve upstream air (i.e., air within the intercooler) vary within the above-mentioned short period of time. Accordingly, in the internal combustion engine having the supercharger and the intercooler, it is difficult for the above-mentioned conventional apparatus to accurately estimate the cylinder intake-air quantity at the future point in time later than the present point in time.